Include m17n-core.h instead of m17n.h.

Include m17n-core.h instead of m17n.h.
(Mexist): New variable.
(enum FontLayoutCmdRuleSrcType): New enum SRC_EXIST.
(FontLayoutCmdRule): New member src.exist.
(parse_otf_command): Handle gsub_count and gpos_count separately.
(load_otf_command): Adjusted for the change of MFLTOtfSpec.
(load_command): Handle the command "exist".
(free_flt_command): Adjusted for the change of MFLTOtfSpec.
(run_rule): Handle the case SRC_EXIST.
(run_otf): Adjusted for the change of MFLTOtfSpec.
(run_command): Check the range of FROM.
(check_otf_spec): Delete it.
(m17n_init_flt): Call m17n_init_core instead of m17n_init.
Initialize Mexist.
(m17n_fini_flt): Call m17n_fini_core instead of m17n_fiini.
(mflt_find): Call font->check_otf instead of check_otf_spec.
(mflt_run): Keep the metrics in 26.6 fixed point.